This course will explore five major mass extinctions that occurred on earth over the past 450 million years of its 4.5-billion-year geologic history, plus the sixth extinction currently underway, due to climate change and other human-caused impacts. We will examine the fascinating circumstances that led to their respective apparent causes, and how those circumstances might relate to current climate change impacts on the future of life on earth. The most recent of the past mass extinctions brought an end to the dinosaurs and most other land animals at the end of the Cretaceous Period 66 million years ago -- a catastrophe caused by the impact of a massive asteroid near the Yucatan Peninsula. The other four past extinctions, some of which were even more extensive, are less well known. Most of them were related to rapid and large increases in atmospheric CO2, and accompanied by rapid global warming. Current alarming climate change trends and causes will also be explored in some detail. The course content will be based in part on two recent well-researched books as well as on the instructor's additional research. The course will be presented at an educated lay-person's level.